Lighting emitting device employing nanowire phosphors

ABSTRACT

Disclosed is a light emitting device employing nanowire phosphors. The light emitting device comprises a light emitting diode for emitting light having a first wavelength with a main peak in an ultraviolet, blue or green wavelength range; and nanowire phosphors for converting at least a portion of light having the first wavelength emitted from the light emitting diode into light with a second wavelength longer than the first wavelength. Accordingly, since the nanowire phosphors are employed, it is possible to reduce manufacturing costs of the light emitting device and to reduce light loss due to non-radiative recombination.

RELATED APPLICATIONS

This application is a U.S. national phase application of PCTInternational Application No. PCT/KR2006/002147, filed Jun. 5, 2006,which claims priority of Korean Patent Application No. 2005-062298,filed Jul. 11, 2005, the contents of which are incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a light emitting device, and moreparticularly, to a light emitting device capable of reducing light lossdue to wavelength conversion by employing nanowire phosphors.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,812,500 discloses a light emitting device comprising aGaN-based, particularly, AlInGaN-based light emitting diode capable ofemitting ultraviolet or blue light, and phosphors absorbing a portion oflight emitted from the light emitting diode and emitting light withconverted wavelengths, thereby implementing polychromatic light, e.g.,white light. Since such a white light emitting device uses asingle-wavelength light source as a light source, its structure is verysimple as compared with a white light emitting device using a pluralityof light sources for different wavelengths.

Examples of phosphors used in the white light emitting device include anYAG:Ce phosphor using Ce³⁺ as an activator, an orthosilicate phosphorrepresented by Sr₂SiO₄:Eu using Eu²⁺ as an activator, and a thiogallatephosphor such as CaGa₂S₄:Eu.

These phosphors are generally prepared in a powder form through a solidstate reaction method, and high-purity raw materials and strictstoichiometric compositions are required to synthesize these phosphors.Particularly, heat treatment at a high temperature of 1300° C. or moreis required to synthesize YAG:Ce. This raises costs of the phosphors,leading to increase in manufacturing costs of the white light emittingdevice.

Further, since each of these powdered phosphors has many traps therein,it is likely to cause non-radiative recombination. Such non-radiativerecombination leads to light loss, resulting in considerable reductionof wavelength conversion efficiency.

An object of the present invention is to provide a light emitting deviceemploying phosphors that can be easily prepared to have high purity.

Another object of the present invention is to provide a light emittingdevice employing phosphors capable of reducing light loss due tonon-radiative recombination.

To achieve these objects of the present invention, a light emittingdevice according to an embodiment of the present invention comprises alight emitting diode for emitting light having a first wavelength with amain peak in an ultraviolet, blue or green wavelength range; andnanowire phosphors for converting at least a portion of light having thefirst wavelength emitted from the light emitting diode into light with asecond wavelength longer than the first wavelength.

The nanowire phosphors can reduce the number of traps as compared withconventional powdered phosphors, resulting in reduction of light lossdue to non-radiative recombination.

Here, the term “nanowire” means a structure having a length relativelylarger than the diameter thereof and having a nano-scale diameter ofless than 1 μm.

The nanowire phosphor may be a nanowire made of ZnO, ZnO doped with Ag,ZnO doped with Al, Ga, In and/or Li, ZnO:Cu,Ga, ZnS:Cu,Ga,ZnS_(1-x)Te_(x)(0<x<1), CdS:Mn capped with ZnS, ZnSe, Zn₂SiO₄:Mn, (Ba,Sr, Ca)₂SiO₄:Eu, or a nitride expressed by a general formulaAl_(x)In_(y)Ga_((1-x-y))N (0≦x<1, 0<y<1, 0<x+y≦1).

With proper selection of a composition ratio of the nanowire phosphor,the light having the 2 first wavelength can be converted into the lighthaving the second wavelength in a range of visible light.

Meanwhile, the Al_(x)In_(y)Ga_((1-x-y))N nanowire phosphor may have acomposition ratio varying in a longitudinal direction such that thelight having the second wavelength has at least two main peaks.Accordingly, in addition to the light having the first wavelength,polychromatic light having two or more colors can be implemented usingone kind of nanowire phosphor.

Meanwhile, the nanowire phosphors may be formed on a substrate usingmetal organic chemical vapor deposition (MOCVD), metal organic hydridevapor phase epitaxy (MOHVPE) or molecular beam epitaxy (MBE). There isno specific limitation on the substrate and the substrate may be, forexample, a silicon (Si) substrate. Thereafter, the nanowire phosphorsare separated from the substrate. Thus, the nanowire phosphors can beeasily fabricated, resulting in reduction of manufacturing costs.

A resin such as epoxy or silicone may cover the light emitting diode.The nanowire phosphors may be dispersed within the resin.

Meanwhile, the nanowire phosphor comprises a core nanowire and ananoshell covering the core nanowire. The nanoshell preventsnon-radiative recombination from being produced on the surface of thecore nanowire. To this end, the nanoshell is preferably made of amaterial with a bandgap larger than that of the core nanowire.

Since nanowire phosphors are employed in accordance with the presentinvention, a manufacturing process is simplified to reduce manufacturingcosts, and light loss due to non-radiative recombination is reduced toimprove the efficiency of a light emitting device.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a light emittingdevice according to an embodiment of the present invention.

FIG. 2 is a perspective view of a nanowire phosphor according to anembodiment of the present invention.

FIG. 3 is a sectional view of a nanowire phosphor according to anotherembodiment of the present invention.

FIG. 4 is a perspective view illustrating a method of fabricating thenanowire phosphors according to the embodiments of the presentinvention.

FIG. 5 is a graph showing cathodoluminescence depending on the indiumcontent of an InGaN nanowire phosphor according to an embodiment of thepresent invention.

DETAILED DESCRIPTION

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. Thefollowing embodiments are provided only for illustrative purposes sothat those skilled in the art can fully understand the spirit of thepresent invention. Therefore, the present invention is not limited tothe following embodiments but may be implemented in other forms. In thedrawings, the widths, lengths, thicknesses and the like of elements areexaggerated for convenience of illustration. Like reference numeralsindicate like elements throughout the specification and drawings.

FIG. 1 is a schematic sectional view illustrating a light emittingdevice 10 according to an embodiment of the present invention.

Referring to FIG. 1, the light emitting device 10 comprises a lightemitting diode 13 and nanowire phosphors 20. The light emitting diode 13is an Al_(x)In_(y)Ga_((1-x-y))N (0≦x, y, x+y≦1) based light emittingdiode capable of emitting ultraviolet, blue or green light.Particularly, the light emitting diode 13 may be a light emitting diodecapable of emitting blue light in a range of 420 to 480 nm.

In general, the light emitting diode 13 is provided with two electrodes(not shown) for connection to an external power supply. The electrodesmay be positioned at the same side or opposite sides of the lightemitting diode 13. The electrodes may be electrically connected to leadterminals (not shown) by means of an adhesive or bonding wires (notshown).

The light emitting diode 13 may be positioned within a reflection cup17. The reflection cup 17 reflects light emitted from the light emittingdiode 13 to fall in a desired range of viewing angles, therebyincreasing luminance within a certain range of viewing angles. Thus, thereflection cup 17 has a certain inclined surface depending on a requiredviewing angle.

Meanwhile, the nanowire phosphors 20 are positioned over the lightemitting diode 13 to convert a portion of light emitted from the lightemitting diode 13 into light with a relatively longer wavelength. Atthis time, the phosphors 20 may be dispersed within a transparentmaterial 15. The transparent material 15 covers the light emitting diode13 to protect the light emitting diode 13 from external environment suchas moisture or external force. The transparent material 15 may be epoxyor silicone, and may be positioned within the reflection cup 17 as shownin the figure.

If the light emitting diode 13 emits blue light, the nanowire phosphors20 may be excited by the blue light and emit yellow light. On thecontrary, nanowire phosphors 20 that are excited by blue light andrespectively emit green and red light may be positioned together overthe light emitting diode 13. Meanwhile, the nanowire phosphors 20 may beexcited by blue light and emit both green and red light. Accordingly,blue light emitted from the light emitting diode 13, and yellow light orgreen and red light from the phosphors 20 are mixed so that white lightcan be emitted to the outside.

On the other hand, if the light emitting diode 13 emits ultravioletlight, the nanowire phosphors 20 may be excited by the ultraviolet lightand emit blue and yellow light, or blue, green and yellow light, etc.

Consequently, the nanowire phosphors 20 capable of performing wavelengthconversion of a portion of light emitted from the light emitting diode13 are employed to implement polychromatic light with various colorcombinations.

FIG. 2 is a perspective view illustrating a nanowire phosphor 20according to an embodiment of the present invention.

Referring to FIG. 2, the nanowire phosphor 20 is a structure having alength relatively larger than the diameter thereof and having anano-scale diameter of less than 1 μm. The nanowire phosphor 20 may be ananowire made of ZnO, ZnO doped with Ag, ZnO doped with Al, Ga, Inand/or Li, ZnO:Cu,Ga, ZnS:Cu,Ga, ZnS_((1-x))Te_(x) (0<x<1), CdS:Mncapped with ZnS, ZnSe, Zn₂SiO₄:Mn, (Ba, Sr, Ca)₂SiO₄:Eu; or a nitrideexpressed by a general formula Al_(x)In_(y)Ga_((1-x-y))N (0≦x<1, 0<y<1,0<x+y≦1).

ZnO generally emits light falling in a broad band throughout green andyellow regions, and ZnO doped with Ag emits light falling in bandsdivided into green and yellow regions. Further, ZnO doped with Al, Ga,In and/or Li intensifies emission of green and yellow light. Meanwhile,ZnO:Cu,Ga emits deep green light, and ZnS:Cu,Ga emits blue light.Meanwhile, CdS:Mn capped with ZnS emits yellow light.

It has been known that ZnS_((1-x))Te_(x) is grown on a GaAs or Sisubstrate using a molecular beam epitaxy (MBE) method. ZnS_((1-x))Te_(x)can emit light with a desired wavelength throughout the entire region ofvisible light by adjusting x.

Zn₂SiO₄:Mn has α and β phases, and the α and β phases emit green lightand light in the vicinity of yellow light, respectively. Further, (Ba,Sr, Ca)₂SiO₄:Eu can emit light having various colors in the region ofvisible light by adjusting a composition ratio of Ba, Sr and Ca.

Meanwhile, the nanowire made of a nitride expressed by a general formulaAl_(x)In_(y)Ga_((1-x-y))N (0≦x<1, 0<y≦1, 0<x+y≦1) can emit light havingvarious colors in the range of visible light depending on a compositionratio of Al and In.

Although the nanowire phosphor 20 has the shape of a rod, it is notlimited thereto but may have the shape of a longitudinally curved wire.

FIG. 3 is a sectional view illustrating a nanowire phosphor 30 accordingto another embodiment of the present invention.

Referring to FIG. 3, the nanowire phosphor 30 comprises a core nanowire25 and a nanoshell 23 for enclosing the core nanowire. The core nanowire25 may be made of the same material as the nanowire phosphor 20described above.

Meanwhile, it is preferred that the nanoshell 23 be made of a materialwith a bandgap larger than that of the core nanowire 25. Accordingly,non-radiative recombination produced on the surface of the core nanowire25 is prevented, resulting in more reduced light loss.

For example, in a case where the core nanowire 25 is made ofAl_(x)In_(y)Ga_((1-x-y))N (0≦x<1, 0<y<1, 0<x+y≦1), the nanoshell 25 maybe made of a nitride expressed by a general formulaAl_(x)In_(y)Ga_((1-x-y))N (0≦x≦1, 0≦y<1, 0≦x+y≦1).

FIG. 4 is a perspective view for illustrating a method of fabricatingthe nanowire phosphors 20 according to the embodiments of the presentinvention.

Referring to FIG. 4, the nanowire phosphors 20 are formed on a substrate11. It is not necessary that the substrate 11 should be lattice-matchedwith the nanowire phosphors 20. For example, in a case where thenanowire phosphors 20 are made of Al_(x)In_(y)Ga_((1-x-y))N, it is notnecessary that the substrate 11 should be a sapphire or SiC substrate.Since there is no specific limitation on the substrate 11, the substratemay be an inexpensive silicon (Si) or glass substrate.

The nanowire phosphors 20 may be formed using an MOCVD, MOHVPE or MBEmethod. InGaN nanowires formed on a silicon substrate using an HVPEmethod are disclosed in the inventor's paper entitled “InGaN nanorodsgrown on (111) silicon substrate by hydride vapor phase epitaxy”(Chemical Physics Letters 380 (2003) 181-184), published on Sep. 26,2003.

According to the paper, Ga and In metals are reacted with HCl tosynthesize a precursor of Ga and In, and the precursor is transportedtogether with NH₃ to a (111) silicon substrate region so as to formIn_(x)Ga_(1-x)N nanowire on the silicon substrate. At this time, ananowire having a mean diameter of about 50 nm and a mean length ofabout 10 μm is obtained on the substrate at a temperature of 510° C.Meanwhile, the cathodoluminescence (CL) spectrum of In_(0.1)Ga_(0.9)Nhas a main peak at 428 nm. Since a bandgap becomes smaller as the indiumcontent of InGaN increases, the CL spectrum can be shifted toward alonger wavelength if the indium content is increased.

Trimethylgallium (TMG) and trimethylindium (TMI) may be used as theprecursor of Ga and In. Further, a precursor of Al, e.g.,trimethylaluminum (TMA) may be transported to a silicon substrate regionso as to form Al_(x)In_(y)Ga_((1-x-y))N. Meanwhile, flow rates ofprecursors of Ga, In and Al may be controlled to formAl_(x)In_(y)Ga_((1-x-y))N with various composition ratios, and it isalso possible to form a nitride nanowire with different compositionratios along its length.

After the nanowire phosphors 20 formed on the substrate 11 have beenseparated therefrom, they are used in manufacturing the light emittingdevice (10 in FIG. 1).

Meanwhile, after the nanowire phosphors 20 have been formed on thesubstrate 11, the nanoshells (23 in FIG. 3) for covering the nanowirephosphors 20 may be formed. Accordingly, the nanowire phosphors 30 ofFIG. 3 are formed on the substrate 11, and the nanowire phosphors 20become the core nanowires (25 in FIG. 3).

The nanoshell may also be formed using an MOCVD, MOHVPE or MBE methodand may 1 be grown in situ within the same reactor for use in formingthe core nanowire 25. Specifically, after the core nanowires 25 havebeen formed, residual gas within the reactor is exhausted, andprecursors of Ga and N are supplied again into the reactor at flow ratesof 10 to 200 sccm and 100 to 2,000 sccm, respectively. The temperatureof the reactor may be 400 to 800° C. At this time, the precursors of Aland/or In may be supplied together. Accordingly, the nanoshells 23 madeof Al_(x)In_(y)Ga_((1-x-y))N (0≦x≦1, 0≦y<1, 0≦x+y≦1) for covering thecore nanowires 25 are formed. The nanoshells are made of a material witha bandgap larger than that of the core nanowires. For example, if thecore nanowires are made of InGaN, the nanoshells may be made of GaN.

FIG. 5 is a graph illustrating the CL spectra of nanowire phosphorsamples 33, 35, 37 and 39 each of which comprises an In_(x)Ga_(1-x)Ncore nanowire and a GaN nanoshell. Here, these samples are prepared bychanging the indium content x of InGaN. That is, the indium contents xof the samples 33, 35, 37 and 39 are 0.22, 0.33, 0.40 and 0.55,respectively.

Referring to FIG. 5, since the bandgap of InGaN becomes smaller as theindium content of the core nanowire increases, the wavelength of the CLspectrum is shifted toward a longer wavelength. Meanwhile, the samples33, 35, 37 and 39 have main peaks in wavelength ranges of blue, green,yellow and red, respectively.

Consequently, it is possible to provide nanowire phosphors capable ofemitting light throughout the entire region of visible light bycontrolling the indium content of core nanowires.

1. A light emitting device, comprising: a light emitting diode to emitlight having a first wavelength with a main peak in at least onewavelength range selected from ultraviolet, blue and green wavelengthranges; and nanowire phosphors to convert at least a portion of lighthaving the first wavelength emitted from the light emitting diode intolight with a second wavelength longer than the first wavelength, whereinthe nanowire phosphor is a nanowire comprising a nitride expressed by ageneral formula Al_(x)In_(y)Ga_((1-x-y))N (0≦x<1, 0<y<1, 0<x+y≦1), andwherein the Al_(x)In_(y)Ga_((1-x-y))N nanowire phosphor has acomposition ratio varying in a longitudinal direction such that thelight having the second wavelength has at least two main peaks.
 2. Thelight emitting device as claimed in claim 1, wherein the nanowirephosphor is at least one nanowire selected from the group consisting ofnanowires made of ZnO, ZnO doped with Ag, ZnO doped with at least oneelement selected from Al, Ga, In and Li, ZnO:Cu,Ga, ZnS:Cu,Ga,ZnS_((1-x))Te_(x) (0<x<1), CdS:Mn capped with ZnS, ZnSe, Zn₂SiO₄:Mn, and(Ba, Sr, Ca)₂SiO₄:Eu.
 3. The light emitting device as claimed in claim1, further comprising a resin covering the light emitting diode, whereinthe nanowire phosphors are dispersed within the resin.
 4. The lightemitting device as claimed in claim 1, wherein the nanowire phosphorsare prepared by forming them on a substrate using an MOCVD, MOHVPE orMBE method and separating them from the substrate.
 5. The light emittingdevice as claimed in claim 4, wherein the substrate is a siliconsubstrate.
 6. A light emitting device, comprising: a light emittingdiode to emit light having a first wavelength with a main peak in atleast one wavelength range selected from ultraviolet, blue and greenwavelength ranges; and nanowire phosphors to convert at least a portionof light having the first wavelength emitted from the light emittingdiode into light with a second wavelength longer than the firstwavelength, wherein the nanowire phosphor comprises a core nanowire anda nanoshell for covering the core nanowire, and wherein the corenanowire comprises a nitride expressed by a general formulaAl_(x)In_(y)Ga_((1-x-y))N (0≦x<1, 0<y<1, 0<x+y≦1); the nanoshell is madeof a nitride expressed by a general formula Al_(x)In_(y)Ga_((1-x-y))N(0≦x≦1, 0≦y<1, 0≦x+y≦1); and the nitride forming the nanoshell has abandgap larger than that of the nitride forming the core nanowire. 7.The light emitting device as claimed in claim 6, wherein the corenanowire has a composition ratio varying in a longitudinal directionsuch that the light having the second wavelength has at least two mainpeaks.
 8. The light emitting device as claimed in claim 6, wherein thenanowire phosphor is at least one nanowire selected from the groupconsisting of nanowires made of ZnO, ZnO doped with Ag, ZnO doped withat least one element selected from Al, Ga, In and Li, ZnO:Cu,Ga,ZnS:Cu,Ga, ZnS_((1-x))Te_(x) (0<x<1), CdS:Mn capped with ZnS, ZnSe,Zn₂SiO₄:Mn, and (Ba, Sr, Ca)₂SiO₄:Eu.
 9. The light emitting device asclaimed in claim 6, further comprising a resin covering the lightemitting diode, wherein the nanowire phosphors are dispersed within theresin.
 10. The light emitting device as claimed in claim 6, wherein thenanowire phosphors are prepared by forming them on a substrate using anMOCVD, MOHVPE or MBE method and separating them from the substrate. 11.The light emitting device as claimed in claim 10, wherein the substrateis a silicon substrate.